1. Field of the Invention
The present invention relates to an image capture apparatus such as a video camera.
2. Description of the Related Art
Conventionally, an image capture apparatus includes a lens used to optically form an object image. Such lenses normally suffer, for example, various aberrations and marginal illumination drops, which results in distortions and density gradients in output images, thus causing image quality deterioration.
These aberrations and marginal illumination drops change depending on, for example, focal lengths, object distances, and aperture values (which will be referred to as optical parameters hereinafter). For this reason, in some conventional image capture apparatuses, the optical characteristics of a lens are stored in advance in a memory, and optical characteristics corresponding to optical parameters are acquired from the memory to correct a captured image (Japanese Patent Laid-Open No. 2000-069343).
However, Japanese Patent Laid-Open No. 2000-069343 does not particularly disclose an image sensor drive method. That is, when an image is captured using an image sensor, of a line-sequential scanning type, represented by, for example, a CMOS sensor, accumulation start and end time differences for respective lines of an image sensor caused by that scanning type are not taken into consideration.
FIG. 12 illustrates an accumulation state of a CMOS sensor and a variation state of a focal length as an optical parameter. A vertical sync signal 801 is decided based on an image capture method of an image capture apparatus, and is periodically generated at 59.94 Hz in case of, for example, National Television System Committee (NTSC) standard. A chart 802 depicts driving of a zoom lens. By plotting the time on the abscissa and the focal length on the ordinate, the chart 802 represents a focal length variation state caused upon driving of the zoom lens.
Target focal lengths 803, 804, and 805 of the zoom lens are decided for respective frames. In this case, assume that the target focal length is decided once per vertical sync period, and the zoom lens is driven to the target focal length during one vertical sync period. Charts 806 and 807 represent read drive operations of an image sensor of a line-sequential scanning type represented by a CMOS sensor. Also, charts 808 and 809 represent drive operations required to reset charges accumulated on the image sensor.
Lines 810, 811, and 812 represent some periods of a charge accumulation state on the image sensor, and each period starts when charges are reset by the reset drive operation 808 and ends when charges are read out by the read drive operation 807. Focusing attention on lines of the image sensor at that time, the first line 810 and Y-th line 812 have different times when charges are accumulated, that is, different accumulation start and end times, as can be seen from FIG. 12. However, Japanese Patent Laid-Open No. 2000-069343 does not consider such differences, and corrects the first to Y-th lines by acquiring optical characteristics based on the focal length 804 as the optical parameter at periods of the vertical sync signal. For this reason, a problem of deterioration of correction precision is posed depending on lines of the image sensor. This problem is not limited to a case in which the zoom lens is driven, but the same problem is posed when a focus lens is driven.